Most of the polymerizable urethane oligomers are urethane acrylate. By having various skeleton structures with good balance, it can easily provide many performances like plasticity, bending property, flexibility, toughness, solvent resistance, and abrasion resistance. Due to such reasons, the polymerizable urethane oligomers are widely used in many fields including cohesives, coatings, and inks. As for the urethane acrylate, by reacting first polyol with polyisocyanate in general, polyurethane having a hydroxyl group or an isocyanate group at both ends is obtained. After that, according to a further reaction between the polyurethane and acrylate containing a hydroxyl group or acrylate containing an isocyanate group, urethane acrylate is synthesized (Patent Literatures 1 to 3). Also suggested is a synthetic method in which acrylate containing a hydroxyl group is reacted with polyisocyanate and the reaction product is linked to polyol terminal (Patent Literature 4).
Urethane acrylate has a radical polymerizable acrylate group. Accordingly, similar to monofunctional acrylate, polyfunctional acrylate, and epoxy acrylate, it is widely used as a constitutional component of an active energy ray curable resin composition. Meanwhile, in accordance with an increased blending amount of urethane acrylate, the curing property of an entire composition decreases. As such, there is a problem that, when it is applied as a resin composition on a substrate and cured by irradiation of an active energy ray like ultraviolet ray, stickiness remains on a surface of a coating film, and thus it is difficult to have a tack free state. Furthermore, when an elastic layer with thickness is formed as a sealing material between a solar battery module and a frame, or a sealing material for semiconductors, liquid crystals, and LED, non-cured components are present in a large amount inside the layer. As such, sufficiently satisfying performances cannot be obtained. There is also a contamination problem which is caused by bleed out of non-cured components over time. As such, to solve the sticky residuals on a surface and non-curing inside a thick layer, combined use with monofunctional or polyfunctional acryl monomer or photosensitizer is reported. For example, in Patent Literature 1, a photocurable resin composition which is obtained by blending urethane acrylate with polybutadiene skeleton with 20 to 80% of monofunctional acrylate and 2% of a sensitizer is suggested. However, in most cases, solubility of the urethane acrylate with polybutadiene skeleton in an acrylate monomer is insufficient due to the influence of a polybutadiene structure having high hydrophobicity. Meanwhile, there is a problem that flexibility specific to the polybutadiene skeleton is deteriorated in accordance with use of the polyfunctional acrylate. There is also a problem that, as an acryl monomer is blended, it becomes difficult to maintain the cure shrinkage rate at a low level after irradiation of ultraviolet (UV) ray.
Furthermore, to improve overall the active energy ray curing property of a urethane acrylate resin, in particular, the hardness and surface stickiness of a cured product, an oligomer as an adduct of urethane acrylamide is suggested (Patent Literatures 5 to 8). By modifying the polymerizable group from an acrylate group to an acrylamide group, the ultraviolet ray curing property is enhanced, and stickiness on a surface of a cured film and hardness of a cured product are improved. However, no mention is made at all regarding the solubility for general purpose organic solvents and acrylic monomers, and transparency, scratch resistance, abrasion resistance, and curing shrinkage resistance of an obtained cured film.